The 1969 El Camino had points ignition originally.
If you still have the points ignition you need to set the point gap first. After starting the engine and getting it to 'normal' operating temps, you will need to check and set the dwell with a dwell meter. The adjustment is made using a small allen wrench thru the sliding window in the distributor cap. I don't recall the specs for the dwell. You will have to get the specs for the dwell. Check the automotive section in the library.
After setting the point gap and the dwell, hook up the timing light and set the timing as posted above. Should be in the 10-12 degree range at idle in NEUTRAL for the stock engine.
If you have a non-computer controlled GM HEI, setting the timing will be the same except there are no points to adjust and no dwell to adjust.
Always check/adjust the timing with the vacuum advance dis-connected and plugged. Re-connect AFTER checking/setting the timing. This will be the same with points or HEI ignition. Use full manifold vacuum for the advance canister. When re-connecting the vacuum advance hose the idle will increase. Re-adjust the idle after re-connecting the vacuum hose. The idle should be about 650-750 RPM in DRIVE if an automatic and in NEUTRAL if a standard shift.